1. Field
The present invention relates to techniques for charging a battery. More specifically, the present invention relates to a method and an apparatus for charging a set of battery cells which are coupled together in series.
2. Related Art
Battery performance is critical to the effective operation of portable computing devices, such as laptop computers. To provide higher supply voltages, banks of battery cells are typically stacked in series inside a battery pack. More specifically, a battery is often constructed from cells in a series-parallel combination. Cells of the same type, but possibly different coulomb capacities, may be connected in parallel to form what is effectively a single, larger capacity cell, called a “bank”. Two or more closely matched banks may be connected in series to provide a higher supply voltage, which may be required for some of the system components, or to meet power efficiency goals.
A series battery is typically charged with reference to a voltage V across the entire battery under the assumption that the cells are well-balanced in capacity and state-of-charge. For example, FIG. 1 illustrates how a charger 102 is used to charge a series battery comprised of banks 104, 106 and 108, which are connected together in series. Note that charger 102 controls the charging process based on a voltage V 110 across the entire series battery. Ideally, if the voltage is V for a battery comprised of m banks connected in series, the voltage of any one bank is V/m. However, in practice there are always discrepancies, so some banks are at a voltage higher than V/m and others are lower. If the discrepancies are uncontrolled and the battery is charged according to its total series voltage V, some cells may be overcharged and experience a shortened service life.
Some systems provide for rebalancing to reduce these discrepancies. However, they do so gradually, and only for static, relaxed conditions. Even with perfect static balance, there can be a significant imbalance in the bank voltages during charging if the cells have differing transport properties, which may be caused, for example, by manufacturing variability or temperature differences.
Hence, what is needed is a technique for charging a series battery that does not suffer from the above-mentioned problems.